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Basement configuration of the Jhagadia–Rajpipla profile in the western part of Deccan syneclise, India from travel-time inversion of seismic refraction and wide-angle reflection data
Journal of Asian Earth Sciences 40 (2011) 40–51
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Basement configuration of the Jhagadia–Rajpipla profile in the western part of Deccan syneclise, India from travel-time inversion of seismic refraction and wide-angle reflection data A.S.N. Murty ⇑, P. Koteswara Rao, M.M. Dixit, G. Kesava Rao, M.S. Reddy, B.R. Prasad, D. Sarkar National Geophysical Research Institute (CSIR), Uppal Road, Hyderabad 500 606, India
a r t i c l e
i n f o
Article history: Received 22 September 2008 Received in revised form 4 December 2009 Accepted 3 March 2010
Keywords: Seismic refraction Wide-angle reflection Low velocity zone Deccan syneclise Travel-time inversion Velocity structure
a b s t r a c t 2-D velocity structure up to the basement is derived by travel-time inversion of the first arrival seismic refraction and wide-angle reflection data along the SW–NE trending Jhagadia–Rajpipla profile, located on the western part of Deccan syneclise in the Narmada–Tapti region. The study region is mostly covered by alluvium. Inversion of refraction and wide-angle reflection data reveals four layered velocity structure above the basement. The first two layers with P-wave velocities of 1.95–2.3 km s 1 and 2.7–3.05 km s 1 represent the Recent and Quaternary sediments respectively. The thickness of these sediments varies from 0.15 km to 3.4 km. The third layer with a P-wave velocity of 4.8–5.1 km s 1 corresponds to the Deccan volcanics, whose thickness varies from 0.5 km to 1.0 km. Presence of a low velocity zone (LVZ) below the high velocity volcanic rocks in the study area is inferred from the travel-time ‘skip’ and amplitude decay of the first arrival refraction data and the wide-angle reflection from top of the LVZ present immediately after the first arrival refraction from Deccan Trap layer. The thickness of the low velocity Mesozoic sediments varies from 0.3 km to 1.7 km. The basement with a P-wave velocity of 5.9–6.15 km s 1 lies at a depth of 4.9 km near Jhagadia and shallows to 1.2 km towards northeast near Rajpipla. The results indicate presence of low velocity Mesozoic sediments hidden below the Deccan Trap layer in the western part of the Deccan syneclise. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Mesozoic sedimentary sequences contain about 54% conventional oil and 44% of gas reserves of the world, discovered so far. Geological factors, viz., sea level changes, restricted marine conditions and high rate of biological activity predominantly influenced development of Mesozoic petroleum system. In this context, Mesozoic being an important period of source rock habitat, studies have been carried out in relation with the Indian basins. The Deccan volcanics (Trap) of late Cretaceous to Paleocene period cover large parts of the Indian Peninsula (Fig. 1). The Mesozoic sediments were identified under the Deccan volcanics in Kutch, Saurashtra and Cambay basins from the deep wells drilled by the Oil and Natural Gas Corporation (ONGC) (Roy, 1991; Singh et al., 1997; NGRI, 2000; Dixit et al., 2000). The Mesozoic sediments are also exposed on the flanks of Deccan syneclise, in some parts of Kutch, Saurashtra and Narmada basins (NGRI, 2004; Kaila et al., 1981). The source-rock potential of the Trap covered sequences are similar to the sequences not covered by the Trap. Hence, there is great interest in oil industry to know the thickness of the Trap and subtrappean ⇑ Corresponding author. Tel.: +91 040 23434700; fax: +91 040 23434651. E-mail address: [email protected] (A.S.N. Murty). 1367-9120/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2010.03.015
Mesozoic sediment and their aerial extension. Seismic reflection studies are not much successful because of energy loss due to absorption and poor signal-to-noise (S/N) ratio in the near vertical range. Large impedance contrasts within the volcanic rocks caused by interbeds, breccia and vesicles generate multiples and scattered noise contaminates the primary reflections (Sain et al., 2002; Jarchow et al., 1994; Pujol et al., 1989). It is known that the subtrappean sedimentary formations can be recognized by the presence of low velocity zone (LVZ) in seismic exploration. Refraction surveys are occasionally used in oil exploration, particularly where it can assist in resolving complicated problems in structural geology (Dixit et al., 2000). The seismic first arrivals indicate a clear cut skip under favorable conditions in the arrival time, which defines the presence of LVZ (Whiteley and Greenhalgh, 1979; Lutter et al., 1994; Tewari et al., 1995). The skips in the first arrival refraction travel-time data are identified as a seismic signature of the low velocity sediments present below the high velocity volcanic rocks (Tewari et al., 1995). In the presence of a LVZ, reflection from the top of the LVZ appears prominently, immediately after the first arrival refraction from the layer above the LVZ. The segment length of the overlying high velocity layer; the magnitude of the skip; offset of the underlying high velocity layer in the first arrival refraction data and the velocity of the low velocity zone obtained from
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Fig. 1. Location of seismic refraction profiles shot in the region on geological map of the Deccan syneclise, India. The geological map of the Jhagadia–Rajpipla (SW–NE) profile (A) is blown up.
an independent source are useful parameters to derive the overlying high velocity layer and the low velocity zone thickness (Tewari et al., 1995; Jarchow et al., 1994). Wide-angle reflection data from top and bottom of the LVZ are a good constraint to map the interfaces between Trap and LVZ and LVZ and basement. Integrated geophysical exploration studies using seismic refraction, magnetotellurics, deep resistivity sounding and gravity methods by the National Geophysical Research Institute (NGRI) have successfully delineated subtrapean Mesozoic sediments in Saurashtra and Kutch (NGRI, 1998 and NGRI, 2000) basins. Similar studies for exploration of subtrappean Mesozoic basins were carried out in the Narmada–Tapti region of the Deccan syneclise, sponsored by the Oil Industry Development Board (OIDB). Seismic refraction studies were carried along nine profiles (Fig. 1) and covered a total length of 700 km during 2002–2003. The results of the integrated geophysical studies identified some areas with large thickness of subtrappean sediments and submitted in the form of a technical report (NGRI, 2004) to OIDB. Based on the results of this report, Directorate General of Hydrocarbons (DGH) would like to take up limited number of parametric drill holes in some of the identified areas. Reanalysis and modeling of the seismic data were undertaken along a few seismic lines identifying wide-angle reflection phases from top and bottom of the low velocity layer together with refraction data. 2-D travel-time inversion method of Zelt and Smith (1992) was used to derive the shallow velocity structure including the subtrappean sediment thickness and basement configuration along the profiles. The inversion method provides resolution and uncertainties of estimated model parameters along with the assurance that the data have been fitted according to a leastsquares norm and is capable of quickly analyzing the data while avoiding unnecessary structures (Zelt, 1999). The present paper deals with the refraction and wide-angle reflection data analysis
and processing results along the Jhagadia–Rajpipla (SW–NE) profile. The objectives of the present study are: (i) to determine the thickness of the sediments, Deccan Traps and subtrappean sediments; (ii) delineate the basement configuration providing with measures of resolution and uncertainty of the model parameters.
2. Geologic setting The study region is spread over 40,000 km2 area between the Narmada and Tapti rivers (Fig. 1) in western part of central India. The region is characterized by the Recent alluvium along the Tapti graben towards southeast, Archaean–Neoproterozoic granite gneiss and Paleo-Mesoproterozoic Aravalli/Delhi super group of rocks towards north, Meso-Neoproterozoic Vindhyan sediments towards east and the Cambay basin towards west. The Jhagadia– Rajpipla profile is located in the northwestern part of the Deccan syneclise and to the east of the Cambay basin. The Deccan volcanics represent an episode of volcanism marking the close of the Mesozoic era. The Deccan Traps were encountered at varying depths below the Tertiary sediments ranging in age from the Palaeocene to the Recent in the wells drilled in the Cambay basin (Roy, 1991; Tewari et al., 1995). Upper Jurassic to Middle Cretaceous Bagh and Lameta beds, representing the Mesozoic sediments underlying the Deccan Traps, is exposed to the east of the Cambay basin near Rajpipla (Fig. 1). The presence of sporadic outcrops of Jurassic and Cretaceous sediments on the margins of the Cambay basin lead to the possibility of a thicker marine sequence within the basin below the Deccan Traps. Deccan Traps also cover the whole of Narmada valley and adjoining areas with exposures of Mesozoic sediments towards north. The Deccan Traps show a marked thickening towards the west coast of India, suggesting that
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the center of eruption lies towards west (NGRI, 2004; Mahadevan, 1994). White and Mckenzie (1989) suggest that the major extrusion took place near the west coast of India around 65 Ma (Courtillot et al., 2000). Deccan Traps covered an area of 5,00,000 km2, excluding the portion down faulted into the Arabian Sea to the west. Mesozoic sediments belonging to the Bagh group are well developed in parts of the Narmada valley. These sediments are exposed mainly along the margins of the Deccan Traps, occupying a position between the Traps and the underlying Precambrian metamorphic rocks. The Mesozoic sediments were uplifted, faulted, intruded, and covered by the Deccan volcanic flows during the late Cretaceous.
3. Refraction experiment and the data analysis Seismic refraction data were acquired along the Jhagadia– Rajpipla profile from five different shot points using 120-channel Radio Frequency Telemetry (RFT) systems (Fig. 1). The data were recorded with 100 m receiver interval (10 Hz natural frequency) and 8–10 km shot interval on a spread length of 11.9 km using 2 ms sampling interval. The data from each shot point were recorded to a distance of 48 km (four spreads) to get an adequate coverage for basement configuration. The seismic recording system provided a wide dynamic range >110 db and a wide band of frequencies up to 125 Hz. In general, a shot hole drilled, to a depth of 20–25 m and loaded with 50 kg high energy explosive, was used as a seismic source. The charge size is increased in steps of 50 kg progressively as the shot-receiver distance increased. The large charges were accommodated in multiple holes, with either rectangular or triangular pattern of holes, with individual shot holes spaced at 10 m within the pattern. The data were recorded during the night to minimize cultural noise thereby increasing S/N ratio. The data were recorded simultaneously in the analog form on photographic paper and digital (SEG-D) form on magnetic tapes and converted to the SEG-Y format. Preprocessing includes trace editing, merging, frequency and velocity filtering using the ProMax seismic processing package on Sun work station. The seismic data are presented in the form of shot gathers (record sections) for various shot points to see the entire seismic data at a glance and identify the phases. Filtered record sections with a reduced travel-time are the best means of presenting the seismic refraction data. The seismic record sections plotted with a reduction velocity of 6 km s 1 are presented in Figs. 2a–2e. Amplitudes are trace normalized, which means that the maximum amplitude is the same for each trace. The records are band pass filtered in the frequency range of 10–24 Hz. In all the record sections (Figs. 2a–2e) Pi indicates the first arrival refraction and Pi the wide-angle reflection data. The overall signal-to-noise ratio of the first arrivals and reflection data as observed on record sections is good. The record sections for SPs 1, 2, 3, 4 and 5 are displayed in Figs. 2a–2e respectively. For the remainder of this paper, ‘‘offset” will refer to the distance between shot and receiver and ‘‘distance” will refer to a location along the profile relative to SP 1 at 0 km. Shot point number followed by SW or NE refer to data recorded to the southwest or northeast of the shot point, respectively. The record section of SP 1 shows only two refraction phases P1 and P2 (Fig. 2a) as the charge detonated was very low due to habitation. The record section of SP 2 (Fig. 2b) shows four refraction phases P1, P2, P3 and P5 as first arrival. From the apparent velocities of the phases (1.98–2.0 and 2.7–2.9 km s 1 respectively) and surface geology exposures, P1 and P2 represent phases from the layers of Recent and Quaternary sediments. The apparent velocities of P3 and P5 phases are 5.1 km s 1 and 6.15 km s 1. These orders of velocities are characteristic of Deccan Traps and granitic basement respectively in this region (Kaila et al., 1981). There is amplitude
decay at about 14 km offset from SP 2 NE (Fig. 2b) and also travel-time skip between the two refracted phases P3 and P5. The amplitude decay in the P3 refracted phase and the time skip in between the P3 and P5 phases indicate the presence of a low velocity zone (LVZ) beneath the Trap layer. This may be due to the fact that the relatively high frequency energy traveling in the relatively thin high velocity layer, overlying a low velocity zone, might have died out before the arrival of low-frequency energy from the basement. The low velocity zone overlying the basement might be responsible for the delay in the travel-time from the basement (Greenhalgh, 1977). The same phenomena of decay of refracted phase and travel-time skip are observed in SPs 3, 4 and 5 (Figs. 2c–2e). The energy of the first arrival refraction data from basement (P5) is relatively poor in most of the record sections probably due to loss of energy to the seismic wave while traveling in the Deccan Trap formation and LVZ. Different gains are used for different phases (low gain for near offset and high gain for far offset record sections) to minimize errors in data picking. Record sections of SP 2 NE, SP 3, SP 4 and SP 5 (Figs. 2b–2e) show wide-angle reflection phase designated as P4 immediately after the first arrival refracted phase P3. The reciprocal travel-times of this phase have been verified while identifying the phase. The prominent wideangle reflection phase immediately after the first arrival Trap refracted phase indicates existence of a large velocity contrast. The indication of a travel-time skip and amplitude decay in the refraction data together with the presence of a prominent wide-angle reflection phase due to large velocity contrast strengthens the assumption of a low velocity zone under the Trap cover. This prominent reflection phase has been identified as a reflection from the interface between the Trap and the LVZ. Similarly there is another reflection phase designated as P5 and identified in SP 4 and SP 5 based on signal strength as there is no reciprocal coverage. This has been interpreted as wide-angle reflection phase from the bottom of the LVZ. It is observed that in the record section of SP 2 (Fig. 2) the refraction phase P3 from the high velocity Deccan Trap has been recorded at 5 km offset from the shot point 2 NE, while the refraction phase P2 recorded up to 10 km towards SW do not show the high velocity refraction phase P3. This indicates that the high velocity Deccan Trap layer must have gone down in SW direction. It is also seen that the offset distance of the refraction phase from basement (P5) (Figs. 2b–2e) decreases progressively towards northeast indicating that the basement depth is shallow towards northeast and deepens towards southwest. The travel-time skip between P3 and P5 phases in the record section of SP 5 (Fig. 2e) is at less than 5 km offset and the magnitude of the skip is small. This indicates that the basement is at very shallow depth near SP 5 and the layers above basement are thin. Again there is a travel-time skip in the basement refracted phase (P5) at about 25 km offset in the record section of SP 5 (Fig. 2e) indicating deepening of the basement towards SW. Similarly Amplitude decay in the Trap refracted phase (P3) and very large travel-time skip between the refracted phases P3 and P4 in the record sections of SP 3 SW and SP 4 SW might be due to the combined result of LVZ beneath the Deccan Traps and deepening of the basement towards southwest. The variation in the magnitude of the skip between the refracted phases P3 and P5 indicates the thickness variation in the LVZ (higher the magnitude larger the thickness) as can seen in the record sections of SP 3 SW and SP 3 NE; SP 4 SW and SP 4 NE. Based on the above observations and also the exposures of Mesozoic sediments near Rajpipla towards the NE of the profile, a low velocity layer correspond to Mesozoic sediments was inferred to lie above the basement and below the Deccan Trap along the profile. As seen from the record sections, the first arrival refraction data (P3) representing the Deccan Trap terminates and the first arrivals at greater offsets are delayed. The termination of diving wave from Deccan Trap occurs when the wave feels the base of
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Fig. 2a. The record section of SP 1 of the profile shows the P1 and P2 refraction phases.
the Trap, consistent with Snell’s law, is refracted into the lowvelocity subtrappean sedimentary section. The offset at which
the direct-wave termination occurs is a function of the velocity structure and thickness of the Trap layer. So the Deccan Trap diving
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Fig. 2b. The record section of SP 2 of the profile shows the P1 and P2 refraction phases from Recent and Quaternary and P3 and P5 refraction phases from Deccan Trap and basement respectively. Amplitude decay for the P3 phase is observed at 14 km distance. The travel-time ‘skip’ between P3 and P5 indicates the presence of a low velocity zone (LVZ) beneath the Trap layer. The absence of P3 towards SW and its presence at 5 km offset towards NE indicates deepening of the Trap rock towards southwest. Wideangle reflection phase P4 from top of the LVZ is shown.
Fig. 2c. Same as Fig. 2b for SP 3 showing refraction phases P1, P3 and P5 with travel-time skip between P3 and P5 phases. Wide-angle reflection phase P4 from top of the LVZ is shown.
wave can be used to image the Trap velocity structure. Trap thickness will be obtained from the maximum depth penetration of the diving wave that fits the termination point of the Trap diving wave (Tewari et al., 1995). Again the wide-angle reflection (P4) further constrains the Trap layer thickness and top of the LVZ, thus eliminating the ambiguity involved in identification of the Trap termination point in the refraction data. Similarly the sedimentbasement interface will be constrained using the correct arrival
time evident at large shot-receiver offset and the velocity of the LVZ (Jarchow et al., 1994; Tewari et al., 1995) together with the wide-angle reflection data (P5) from the bottom of the LVZ. 4. Methodology In the present study, first arrival P-wave refraction and wide-angle reflection data are modeled for shallow crustal velocity
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Fig. 2d. Same as Fig. 2b for SP 4 showing refraction phases P1, P3 and P5 with travel-time skip between P3 and P5 phases. Wide-angle reflection phases P4, P5 from top and bottom of the LVZ are shown.
Fig. 2e. The record section for SP 5 showing refraction phases P2, P3 and P5 with travel-time skip between P3 and P5 at 5 km offset indicates presence of LVZ. The skip in the travel-time of basement refraction P5 at about 25 km offset indicates deepening of the basement towards southwest. Wide-angle reflection phases P4, P5 from top and bottom of the LVZ are shown.
model using the travel-time inversion (Zelt and Smith, 1992) method. The inversion scheme is based on model parameterization and a method of ray tracing suited to the forward step of the inverse approach. The model was parameterized by linear interpolation between an irregular grid of depth nodes and upper and lower layer velocity nodes. A smooth layer boundary simulation is used to avoid scattering and focusing of ray paths and to stabilize the inversion. The travel-times and their partial derivatives with respect to velocity and depth nodes are calculated during the ray tracing that uses an efficient numerical solution (Zelt and Ellis,
1988) of the ray tracing equations (Cerveny et al., 1977). The calculated response of the model is compared with the observed data, and the model parameters are updated using the correction vector obtained from the damped least-squares inversion (Zelt and Smith, 1992; Zelt, 1999). The process is repeated until we achieve a satisfactory fit corresponding to a normalized v2 value of nearly 1. However, it is not always possible to obtain v2 = 1 while maintaining acceptable resolution values of model parameters because of data sampling at small-scale heterogeneities that cannot be resolved by modeling.
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5. Modeling procedure We have analyzed the first arrival refraction and wide-angle reflection data to derive the basement configuration and overlying Deccan Trap and sedimentary formations along the profile. Based on the geological information and considerable change in slopes of the refraction phases, first arrival refraction and wide-angle reflection data are picked from record sections of various shot points using the SeisWide software package. The dominant period of the data is around 50 ms, which has been used as the picking uncertainties to the data. This uncertainty assignment is subjective and is based on one period of dominant cycle of the data, as has been used by Sain et al. (2000). The uncertainty of picking seismic data also depends on the signal-to-noise ratio of each phase. However, utmost care has been taken while picking both refraction and reflection data using varied gains for various phases to avoid ambiguous picks. The first arrival refraction data has been plotted on the record sections (Fig. 2) by dots (). There is clear indication on the shot gathers of shot points 2–5 (Figs. 2b–2e), sudden terminations in the Trap diving wave (P3). The offset at which this termination occurs is approximately 14 km in SP 2 (Fig. 2b). This termination point appears to vary substantially from one record section to the other. While modeling the refraction data, we have considered Pwave velocity of 3.5 km s 1 to the low velocity zone (Mesozoic sediments) present beneath the Trap layer. This velocity was obtained from the available well data and from the micro spreads recorded on the Mesozoic exposures in Cambay, Saurashtra and Kutch basins for velocity determination. A uniform velocity gradient of 0.1 km s 1 km 1 for high velocity Trap layer was taken on the basis of the earlier studies in Saurashtra and Kutch basins, since both the gradient and thickness of the layer play an important role in causing the travel-time shift (Tewari et al., 1995). The first arrival refraction data from the sediments, Deccan Trap, basement layer and wideangle reflection from top and bottom of the LVZ are input to the travel-time inversion program RAYINVR (Zelt and Smith, 1992). The modeling approach is one of ‘layer stripping’, whereby successively deeper layers are determined by keeping fixed the velocity and structural information of overlying layers. Initially, the 1-D velocity-depth functions were derived separately for each shot point using the method of Sain and Kaila (1996). An initial 2-D velocity structure was then prepared along the profile by smoothly joining the individual shot point results for further refinement. The travel-times of the first segment (P1) of each shot point at profile distances of 0.8, 9.78, 16.87, 26.28, and 37.00 km along the profile are inverted simultaneously using the 2-D inversion scheme of Zelt and Smith (1992). This determined the velocity variation of the first layer (1.95–2.3 km s 1). The data are fitted with an rms travel-time residual of 56 ms corresponding to a normalized v2 value of 1.260. The interface structure of the first layer (0.30–0.70 km) and velocity variation of the second layer (2.70– 3.05 km s 1) are then calculated by inverting the travel-time data of the second segments of all shots by holding fixed the velocity variation of the first layer already derived. The data are fitted with an rms travel-time residual of 52 ms corresponding to a normalized v2 value of 1.207. The first two layers may correspond to the Recent and Quaternary sediments. To find out the thickness of the third layer (Trap thickness) it is important to pick precisely the diving wave (P3) termination point. After picking the termination points for all the shot points, rays were traced from individual shot points to their respective termination points and the base of the trap layer was determined from the maximum depth penetrated by these rays with a P-wave velocity of 4.8–5.1 km s 1. Then wide-angle reflection data (P4) were used to generate the computed travel-time from the bottom of the Trap layer. Thus the base of the trap layer would be the top of the low velocity zone (3.5 km s 1), has been obtained (0.9–3.9 km) by modeling both
the refraction (P3) and reflection (P4) data. The data are fitted with an rms travel-time residual of 38 ms and normalized v2 value of 0.588 for refraction and rms travel-time residual of 47 ms and normalized v2 value of 0.877 for reflection data. After the Trap velocity and thickness were determined, a simultaneous inversion for basement velocity and LVZ-basement interface position (bottom of the LVZ) was carried out using the refraction data (P5) at large offset and also wide-angle reflection (P5) data. The uncertainty of the basement depth derived from the refraction data due to ambiguity in picking at few offsets in some shot points along the profile has been reduced to a great extent with the reflection data modeling. The basement velocity (5.90–6.15 km s 1) and depth (1.2–4.9 km) variation were obtained with an rms travel-time residual of 48 ms and normalized v2 value of 0.916 and 0.947 for refraction and wide-angle reflection data respectively. To see the variation of the basement depth with velocity of the LVZ, we have modeled the data set assuming a velocity of 4.3 km s 1 for the low velocity sediment layer. It is observed that the depth to the basement is increased by about 500 m with increase in the velocity of LVZ. For the inversion we use the overall damping factor of 1.0, with apriori velocity uncertainty of 0.1 km s 1 and depth uncertainty of 0.1 km. The rays traced through the final velocity model and their travel-time fit are shown in Figs. 3a and 3b. The final model along the profile is displayed in Fig. 4. The model along the profile corresponds to an overall rms travel-time residual of 47 ms and normalized v2 of 0.899. The number of rays traced through the final model, rms travel-time residual and normalized v2 value for various phases corresponding to all shots is shown as modeling results in Table 1. The numerical values of velocities of various layers (Fig. 4) show the lateral variation in velocity. The velocity model is derived based on the ability to trace rays through the final model to almost all observation points and a trade-off between achieving a sufficiently small travel-time residual of the order of data uncertainties and an adequately high parameter resolution. Adding more nodes will typically reduce the travel-time residual, but decrease the parameter resolution. Therefore, a velocity model with few parameters defining a minimum structure model is generally sought. This represents the true structure (O’Leary et al., 1995) and results are free from modeling artifacts. We decided the number of nodes on the basis of the observed and computed traveltime fit within the data uncertainties and avoiding unnecessary structures to the boundary interfaces. The model parameters used in the inversion are shown in Fig. 5.
6. Resolution and uncertainty The most important aspect of model assessment is to provide a measure of resolution and uncertainties of estimated model parameters from the diagonal elements of the resolution and covariance matrices respectively. Generally, resolution values range from 0 to 1 and depend on the relative number of rays sampling each model parameter. The desired fit and resolution are attained within 3–4 iterations with most of the velocity and depth nodes attain resolution greater than 0.8 along the profile (Table 2) indicating that the model is well resolved. To account for the nonlinearity of travel-time inversion and to provide significant insight into the model constraint, a uni-parameter uncertainty test (Zelt and Smith, 1992; Zelt, 1999) is carried out at few selected nodes to provide the upper bounds. For obtaining absolute uncertainty of a model parameter, we perturb its value from that in the final model and hold it fixed while inverting the observed data involving all other model parameters that were determined along with the perturbed parameter in the final model. Increase or decrease in perturbation is continued until the final model fits the observed data. The maximum perturbation of the parameter that allows a
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Fig. 3a. Refraction and reflection rays (top) traced through the final velocity model for shot points 1, 2 and 3 along the profile. Comparison of observed (vertical bars) and theoretical travel-times (line) for various refracted and reflected phases (bottom). The data are plotted in reduced time using a reduction velocity 6.0 km s 1.
comparable fit to the observed data gives an estimate of its absolute uncertainty. Fig. 6 displays the absolute uncertainty of the
velocity node of the basement layer at 20 km and boundary depth of the basement interface at 10 km profile distance along the
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Fig. 3b. Same as Fig. 3a for shot points 4 and 5 along the profile.
profile. This shows the absolute velocity uncertainty lying between 5.75 and 6.0 km s 1 ( 0.15 km s 1 to +0.10 km s 1) and depth uncertainty between 4.68 km and 4.98 km ( 0.18 km to + 0.12 km) corresponding to 50 ms travel-time residual. We have carried out the uncertainty test at few selected nodes to save the modeling time and observed that the overall velocity and depth uncertainties are of the order of ±0.12 km s 1 and ±0.15 km respectively.
7. Results and discussion The basement configuration along the seismic profile has been derived to a depth of about 5.0 km by travel-time inversion of first arrival refraction and wide-angle reflection data (Fig. 4). Alluvium (Recent) outcrops the entire length of the profile and has been recorded as direct wave in the record sections of SPs 1, 2, 3 and 4. There are four layers above the basement. All the layers including the basement show maximum depth below the shot point 1 near Jhagadia in the southwest and shallowing to minimum depth towards northeast near Rajpipla. The first layer P-wave velocity is
ranging between 1.95 and 2.3 km s 1 represents Recent (alluvium) as the formation is exposed along the profile. The thickness of this layer is 300 m near Jhagadia attains maximum thickness of 700 m near SP 2 and gradually thins out towards northeast and disappears near shot point 5. Second layer with a velocity of 2.7–3.05 km s 1 corresponds to the Quaternary sediments, which exhibits maximum thickness of about 2900 m below shot point 1 near Jhagadia. The thickness of this layer rapidly decreases between SP 2 and SP 3 reaching to a value of about 150 m near Rajpipla to the northeast. The third layer with a velocity of 4.8–5.1 km s 1 represents the Deccan Trap formation. The maximum thickness of the Trap layer is 1000 m near SP 3 thinning to about 500 m on either side. Top of the Trap layer is steeply deepens between shot points 2 and 3 because of which, there is huge thickness of Quaternary sediment deposition over the Trap layer southwest of SP 2 near Jhagadia. The steep deepening of the Trap layer between shot points 2 and 3 can be inferred as a fault (Fig. 4). The amplitude decay and travel-time skip in the first arrival refraction data seen in the shot points 2, 3, 4 and 5 (Figs. 2b–2e) indicate the presence of a low velocity zone (LVZ) beneath the Traps. The
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Fig. 4. The P-wave velocity model along the profile. The numbers within the model represent localized velocities (km s 1) of various layers indicating the lateral variation of the velocities. Velocities of the top two layers are 1.95–2.30 and 2.70–3.05 km/s respectively. Inverted triangles at the top show the location of shot points.
Table 1 Modeling results along the profile. Phase
No. of observations picked
Data uncertainty (ms)
RMS residual (s)
Normalized v2 value
No. of rays traced through model
Refra-P1 Refra-P2 Refra-P3 Refra-P5 Reflec-P4 Reflec-P5
104 82 197 239 169 101
50 50 50 50 50 50
0.056 0.052 0.038 0.048 0.047 0.048
1.260 1.207 0.588 0.916 0.877 0.947
98 82 195 239 167 96
Number of data points used: 877. RMS travel-time residual: 0.047. Normalized chi-squared: 0.899.
Fig. 5. Velocity (dots) and depth (squares) nodes representing the model parameterization along the profile.
thickness of low velocity Mesozoic sediments is about 1000 m near Jhadadia at SP 1 and increases to maximum thickness of 1700 m near SP 2. The low velocity sediment thickness at 25 km distance is about 1100 m and thereon gradually shallows to 300 m towards northeast near Rajpipla. The basement with a velocity ranging between 5.9 and 6.15 km s 1 is at a depth of 4.9 km below SP 1 near Jhagadia. The basement depth decreases from 4.6 km near shot point 2 to 2.6 km near shot point 3 and further shows an upward trend, reaching to a value of 1.2 km near Rajpipla towards northeast. The presence of thick low velocity sediments and steep deepening of the basement interface between shot points 2 and 3 (2 km throw) is reflected on record sections of SP 4 SW in the
form of amplitude decay and time skip between P3 and P5 phases. Again record section of SP 5 SW indicates time skip at 25 km offset in the basement refracted phase P5. Based on the refraction data of SP 4 SW and SP 5 a fault can be inferred in the basement between SP 2 and SP 3. The velocity model derived in the present study delineates the Recent, Quaternary, Trap, low velocity Mesozoic sediment thickness and depth to the basement. It also provides resolution and uncertainty values to the model parameters. The profile is crossing Sinor-Valod (N-S) profile near SP 3 and Mavli-Gulkumar (N-S) profile near SP 5. The results of these profiles (NGRI, 2004) nearly correlates the results obtained in the present profile at the cross points. The inferred fault between SP 2 and 3 in the Deccan Trap layer might have played significant role in deposition of the sediments over it. Sastry et al. (1964) opine that the Cambay basin appears to have been formed due to faulting of the Deccan Trap which incidentally also controlled the nature of the structure in the overlying sediments. The Broach–Ankleswar graben, derived from the deep seismic sounding studies along the Mehmadabad– Billimora profile (Kaila et al., 1981) in the Cambay basin, is located to the west of the present profile. The DSS results of the Broach– Ankleswar graben indicate thick Tertiary sediments deposited over an irregular surface of the Deccan Trap. The granitic basement is at a depth of 5.2 km and there is a possible occurrence of low velocity Mesozoic sediments of 1.2 km thick beneath the Deccan Trap layer (Kaila et al., 1981). In the present analysis the basement depth of 4.9 km inferred in the southwest of Jhagadia nearly coincides with
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Table 2 Resolution estimates along the profile. Distance (km) Velocity L1 0.0 L1 10.0 L1 20.0 L2 5.0 L2 10.0 L2 40.0 L3 16.0 L3 26.0 L5 20.0 L5 27.0 Depth L2 2.0 L2 5.0 L2 12.0 L2 16.0 L2 18.0 L2 20.0 L3 12.0 L3 14.0 L3 16.0 L3 25.0 L3 30.0 L3 40.0 L4 12.0 L4 20.0 L4 25.0 L4 30.0 L4 40.0 L5 18.0 L5 20.0 L5 25.0 L5 30.0 L5 35.0
Velocity nodes (km/s)
Velocity resolution
Standard deviation
2.00 1.98 1.96 3.05 2.90 2.90 4.81 4.87 5.90 6.00 Depth nodes (km)
0.81 0.96 0.87 0.82 0.51 0.88 0.55 0.59 0.86 0.89
0.044 0.021 0.036 0.042 0.070 0.035 0.067 0.064 0.038 0.033
0.50 0.68 0.70 0.50 0.40 0.35 1.90 1.50 1.00 0.60 0.50 0.15 2.94 1.77 1.43 1.36 0.88 2.66 2.55 2.55 2.16 1.72
0.82 0.86 0.83 0.61 0.61 0.74 0.79 0.73 0.81 0.91 0.91 0.84 0.75 0.75 0.76 0.64 0.60 0.70 0.80 0.84 0.82 0.87
0.042 0.037 0.042 0.062 0.062 0.052 0.046 0.052 0.044 0.030 0.030 0.040 0.050 0.050 0.049 0.060 0.063 0.055 0.045 0.040 0.042 0.036
(L = layer)
the basement depth estimates in the earlier DSS profile. The low velocity Mesozoic sediments in the present profile are about 1.7 km thick near the shot point 2. The Mesozoic sediments in western India occupy a large portion of Rajasthan, Gujarat and Madhya Pradesh and are mostly marine, thus attracting the attention of the oil industry. On the southeastern margin of the Cambay basin and western part of the Deccan syneclise near Rajpipal, a 600 m thick section of the Cretaceous Bagh beds (Mesozoic) is exposed (Fig. 1). Poddar (1964) had earlier suggested that there is likelihood for the presence of suitable facies and of favorable prospects of hydrocarbon-bearing Mesozoic sediments below the Deccan Traps towards the Ankleswar oil field. From these considerations and the fact that the existence of a relatively thin Trap and about 1.7 km thick Mesozoic sediments near SP 2 in the present profile is significant. Deep seismic sounding (DSS) studies across Narmada–Son lineament mapped Deccan Trap thickness and also inferred subtrappean sediments in the Narmada–Tapti region (Kaila, 1988; Sridhar and Tewari, 2001). The present study area lies in between two DSS profiles along Mehmadabad–Billimora in the west and Thuadara–Sindad towards the east. Along the Thuadara–Sindad profile the thickness of the Deccan Trap is found to be around 900 m and south of Sendhwa up to Tapti river the thickness of Mesozoic sediments is around 1.9 km (Kaila, 1988). Sridhar and Tewari (2001) in their reinterpreted seismic results along Thuadara–Sindad profile delineated a graben between the rivers Narmada and Tapti. Within the graben, depth to the basement is 5000–5500 m between Sendhwa and Tapti and contains 1000–2800 m thick Mesozoic sediments under a thick (2000–2700 m) cover of the Deccan Trap. From the
Fig. 6. (a) Root-Mean-Square (rms) travel-time residual as a function of velocity perturbation for the velocity node of the basement layer at 20 km profile distance. The absolute velocity uncertainty lies between 5.75 and 6.0 km s 1 ( 0.15 to +0.1 km/s) corresponding to 50 ms travel-time residual. (b) Root-Mean-Square travel-time residual as a function of depth perturbation for the depth node of the basement interface at 10 km profile distance. The absolute depth uncertainty lies between 4.68 and 4.98 km ( 0.18 to +0.12 km) corresponding to 50 ms travel-time residual.
Isopach contour maps of the Mesozoic sediments in the Narmada– Tapti region, Kaila (1988) hypothesis a Mesozoic basin in the Narmada–Tapti area formed a part of larger Mesozoic sea that extended in the shape of an arc from Sanawad-Mahan through Barwani-Sindad, Ankaleswar-Surat, Navibandar-Amreli, Kutch, Rajasthan, Sind and up to Salt Range. The Trap and subtrappean sediment thickness derived in the present study along the profile in the Narmada–Tapti region of the western Deccan syneclise nearly correlates with the Trap and Mesozoic sediment thickness derived in the Narmada–Tapti region by Kaila et al. (1981) and Kaila (1988) from the DSS data and confirm the existence of Mesozoic basin in the Deccan syneclise region.
8. Conclusions The present study images the hidden low velocity Mesozoic sediments lying below the high velocity Deccan Trap volcanic rocks in the Narmada–Tapti region of the western part of Deccan syneclise. The velocity model derived along the profile is well constrained providing model parameter resolution and uncertainties. The first two layers with P-wave velocities of 1.9–2.3 km s 1 and 2.7–3.05 km s 1 are Recent and Quaternary sediments with a thickness variation of 300–700 m and 150–2900 m respectively. The Trap layer (velocity 4.8–5.1 km s 1) thickness varies from 500 to 1000 m and the low velocity Mesozoic sediment thickness ranges from about 300 m to 1700 m along profile. Travel-time skip and amplitude decay of the first arrival refraction together with the wide-angle reflection data from top and bottom of LVZ have been used to derive the thickness of the low velocity zone. The basement (5.9–6.15 km s 1 velocity) lies at a depth of 4.9 km approximately near Jhagadia and shallows to about 1.2 km near Rajpipla. The thick low velocity Mesozoic sediments present below the relatively thin Deccan Traps near SP 2 along the profile is an important zone
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for further hydrocarbon exploration work in the Narmada–Tapti region of the western part of Deccan syneclise. Acknowledgements We are thankful to the Director, National Geophysical Research Institute (NGRI) Hyderabad, for his kind permission to publish this paper. We also thank the Oil Industry Development Board (OIDB), Govt. of India for supporting the project in the Deccan syneclise. We gratefully acknowledge and thank the CSS crew of NGRI for successfully completing the fieldwork along the profiles in the Deccan syneclise. Thanks are to the two anonymous reviewers and our colleague Dr. V. Vijaya Rao for their valuable suggestions to improve the manuscript. We also thank S/Shri. V. Rajasekhar, M. Sankaraiah and B.P.S Rana for drafting the figures. References Cerveny, V., Molotkov, I.A., Psencik, I., 1977. Ray Method in Seismology. University of Karlova, Prague. Courtillot, V., Gallet, Y., Rocchia, R., Feraud, G., Robin, E., Hofman, C., Bhandari, N., Ghevariya, Z.G., 2000. Cosmic Markers, 40Ar/39Ar Dating and Paleomagnetism of the KT Sections in the Anjar Area of the Deccan Large Igneous Province, EPSL, vol. 182, pp. 137–156. Dixit, M.M., Satyavani, N., Sarkar, D., Khare, P. Reddy, P.R., 2000. Velocity Inversion in the Lodhika Well Area, Saurashtra Peninsula, Western India, First Break, vol. 18, pp. 499–504. Greenhalgh, S.A., 1977. Comments on ‘The hidden layer problem in seismic refraction work’. Geophysical Prospecting 25, 179–181. Jarchow, C.M., Catchings, R.D., Lutter, W.J., 1994. Large explosive source, widerecording aperture, seismic profiling on the Columbia Plateau, Washington. Geophysics 59, 259–271. Kaila, K.L., 1988. Mapping the thickness of Deccan Trap flows in India from DSS studies and inferences about a hidden Mesozoic Basin in the Narmada–Tapti Region. The Memoir of the Geological Society of India (10), 91–116. Kaila, K.L., Krishna, V.G., Mall, D.M., 1981. Crustal structure along Mehmadabad– Billimora profile in the Cambay basin, India, from Deep Seismic Soundings. Tectonophysics 76, 99–130. Lutter, W.J., Catchings, R.D., Jarchow, C.M., 1994. An image of the Columbia Plateau from inversion of high-resolution seismic data. Geophysics 59, 1278–1289. Mahadevan, T.M., 1994. Deep continental structure of India: a review. Memoir Geological Society India 28, 239–301. NGRI Technical Report, 1998. Integrated Geophysical Studies for Hydrocarbon Exploration, Saurashtra, India, NGRI Technical Report No. NGRI-1998-Exp-239.
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NGRI Technical Report, 2000. Integrated Geophysical Studies for Hydrocarbon Exploration, Kutch, India, NGRI Technical Report No. NGRI-2000-Exp-296. NGRI Technical Report, 2004. Integrated Geophysical Studies for Hydrocarbon Exploration, Deccan Syneclise, India, NGRI Technical Report No. NGRI-2003Exp-404. O’ Leary, D.M., Ellis, R.M., Stephenson, R.A., Lane, L.S., Zelt, C.A., 1995. Crustal structure of the northern Yukon and Mackenzie delta, north-western Canada. Journal of Geophysical Research 100 (B7), 9905–9920. Poddar, M., 1964. Mesozoic of western India - their geology and possibilities. In: 22nd Int. Geol. Congr., New Delhi, Sect. I-Part I. Geology of Petroleum, 126– 143. Pujol, J., Fuller, B.N., Smithson, S.B., 1989. Interpretation of a vertical seismic profile conducted in the Columbia Plateau basalts. Geophysics 54, 1258–1266. Roy, T.K., 1991. Structural styles in southern Cambay basin, India and role of Narmada geofracture in formation of giant hydrocarbon accumulation. ONGC Bulletin 27, 15–56. Sain, K., Kaila, K.L., 1996. Interpretation of first arrival travel-times in seismic refraction work. Pure and Applied Geophysics 147, 181–194. Sain, K., Bruguier, N., Murty, A.S.N., Reddy, P.R., 2000. Shallow velocity structure along the Hirapur–Mandla profile in central India, using travel-time inversion of wide-angle seismic data and its tectonic implications. Geophysics Journal International 142, 505–515. Sain, K., Zelt Colin, A., Reddy, P.R., 2002. Imaging of subvolcanic Mesozoics in the Saurashtra peninsula of India using travel-time inversion of wide-angle seismic data. Geophysics Journal International 150, 820–826. Sastry, V.L.N., Awasthi, A.M., Mahapatra, P., Mool Chand, Subrata Ray, Kaila, K.L., 1964. Seismic exploration in Cambay basin. In: 22nd Int. Geol. Congr., New Delhi, Part II-2. Geological results of Applied Geophysics 292–306. Singh, D., Alat, C.A., Singh, R.N., Gupta, V.P., 1997. Source Rock Characteristics and Hydrocarbon generating potential of Mesozoic sediments in Lodhika area, Saurashtra basin, Gujarat, India. In: Proceedings Second Int. Pet. Conf. & Exbn. PETROTECH-97, New Delhi, pp. 205–220. Sridhar, A.R., Tewari, H.C., 2001. Existence of a sedimentary graben in the western part of Narmada zone: seismic evidence. Journal of Geodynamics 31, 19–31. Tewari, H.C., Dixit, M.M., Murty, P.R.K., 1995. Use of traveltime skips in refraction analysis to delineate velocity inversion. Geophysical Prospecting 43, 793– 804. White, R., Mckenzie, D., 1989. Magmatism at rift zones: the generation of volcanic continental margins and flood basalts. Journal of Geophysical Research 94, 7685–7729. Whiteley, R.J., Greenhalgh, S.A., 1979. Velocity inversion and the shallow seismic refraction method. Geoexploration 17, 125–141. Zelt, C.A., 1999. Modeling strategies and model assessment for wide-angle seismic travel-time data. Geophysics Journal International 139, 183–204. Zelt, C.A., Ellis, M., 1988. Practical and efficient ray tracing in two-dimensional media for rapid travel-time and amplitude forward modeling. Canadian Journal of exploration Geophysics 24, 16–31. Zelt, C.A., Smith, R.B., 1992. Seismic travel-time inversion for 2-D crustal velocity structure. Geophysics Journal International 108, 16–34.
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Basement configuration of the Jhagadia–Rajpipla profile in the western part of Deccan syneclise, India from travel-time inversion of seismic refraction and wide-angle reflection data A.S.N. Murty ⇑, P. Koteswara Rao, M.M. Dixit, G. Kesava Rao, M.S. Reddy, B.R. Prasad, D. Sarkar National Geophysical Research Institute (CSIR), Uppal Road, Hyderabad 500 606, India
a r t i c l e
i n f o
Article history: Received 22 September 2008 Received in revised form 4 December 2009 Accepted 3 March 2010
Keywords: Seismic refraction Wide-angle reflection Low velocity zone Deccan syneclise Travel-time inversion Velocity structure
a b s t r a c t 2-D velocity structure up to the basement is derived by travel-time inversion of the first arrival seismic refraction and wide-angle reflection data along the SW–NE trending Jhagadia–Rajpipla profile, located on the western part of Deccan syneclise in the Narmada–Tapti region. The study region is mostly covered by alluvium. Inversion of refraction and wide-angle reflection data reveals four layered velocity structure above the basement. The first two layers with P-wave velocities of 1.95–2.3 km s 1 and 2.7–3.05 km s 1 represent the Recent and Quaternary sediments respectively. The thickness of these sediments varies from 0.15 km to 3.4 km. The third layer with a P-wave velocity of 4.8–5.1 km s 1 corresponds to the Deccan volcanics, whose thickness varies from 0.5 km to 1.0 km. Presence of a low velocity zone (LVZ) below the high velocity volcanic rocks in the study area is inferred from the travel-time ‘skip’ and amplitude decay of the first arrival refraction data and the wide-angle reflection from top of the LVZ present immediately after the first arrival refraction from Deccan Trap layer. The thickness of the low velocity Mesozoic sediments varies from 0.3 km to 1.7 km. The basement with a P-wave velocity of 5.9–6.15 km s 1 lies at a depth of 4.9 km near Jhagadia and shallows to 1.2 km towards northeast near Rajpipla. The results indicate presence of low velocity Mesozoic sediments hidden below the Deccan Trap layer in the western part of the Deccan syneclise. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Mesozoic sedimentary sequences contain about 54% conventional oil and 44% of gas reserves of the world, discovered so far. Geological factors, viz., sea level changes, restricted marine conditions and high rate of biological activity predominantly influenced development of Mesozoic petroleum system. In this context, Mesozoic being an important period of source rock habitat, studies have been carried out in relation with the Indian basins. The Deccan volcanics (Trap) of late Cretaceous to Paleocene period cover large parts of the Indian Peninsula (Fig. 1). The Mesozoic sediments were identified under the Deccan volcanics in Kutch, Saurashtra and Cambay basins from the deep wells drilled by the Oil and Natural Gas Corporation (ONGC) (Roy, 1991; Singh et al., 1997; NGRI, 2000; Dixit et al., 2000). The Mesozoic sediments are also exposed on the flanks of Deccan syneclise, in some parts of Kutch, Saurashtra and Narmada basins (NGRI, 2004; Kaila et al., 1981). The source-rock potential of the Trap covered sequences are similar to the sequences not covered by the Trap. Hence, there is great interest in oil industry to know the thickness of the Trap and subtrappean ⇑ Corresponding author. Tel.: +91 040 23434700; fax: +91 040 23434651. E-mail address: [email protected] (A.S.N. Murty). 1367-9120/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2010.03.015
Mesozoic sediment and their aerial extension. Seismic reflection studies are not much successful because of energy loss due to absorption and poor signal-to-noise (S/N) ratio in the near vertical range. Large impedance contrasts within the volcanic rocks caused by interbeds, breccia and vesicles generate multiples and scattered noise contaminates the primary reflections (Sain et al., 2002; Jarchow et al., 1994; Pujol et al., 1989). It is known that the subtrappean sedimentary formations can be recognized by the presence of low velocity zone (LVZ) in seismic exploration. Refraction surveys are occasionally used in oil exploration, particularly where it can assist in resolving complicated problems in structural geology (Dixit et al., 2000). The seismic first arrivals indicate a clear cut skip under favorable conditions in the arrival time, which defines the presence of LVZ (Whiteley and Greenhalgh, 1979; Lutter et al., 1994; Tewari et al., 1995). The skips in the first arrival refraction travel-time data are identified as a seismic signature of the low velocity sediments present below the high velocity volcanic rocks (Tewari et al., 1995). In the presence of a LVZ, reflection from the top of the LVZ appears prominently, immediately after the first arrival refraction from the layer above the LVZ. The segment length of the overlying high velocity layer; the magnitude of the skip; offset of the underlying high velocity layer in the first arrival refraction data and the velocity of the low velocity zone obtained from
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41
Fig. 1. Location of seismic refraction profiles shot in the region on geological map of the Deccan syneclise, India. The geological map of the Jhagadia–Rajpipla (SW–NE) profile (A) is blown up.
an independent source are useful parameters to derive the overlying high velocity layer and the low velocity zone thickness (Tewari et al., 1995; Jarchow et al., 1994). Wide-angle reflection data from top and bottom of the LVZ are a good constraint to map the interfaces between Trap and LVZ and LVZ and basement. Integrated geophysical exploration studies using seismic refraction, magnetotellurics, deep resistivity sounding and gravity methods by the National Geophysical Research Institute (NGRI) have successfully delineated subtrapean Mesozoic sediments in Saurashtra and Kutch (NGRI, 1998 and NGRI, 2000) basins. Similar studies for exploration of subtrappean Mesozoic basins were carried out in the Narmada–Tapti region of the Deccan syneclise, sponsored by the Oil Industry Development Board (OIDB). Seismic refraction studies were carried along nine profiles (Fig. 1) and covered a total length of 700 km during 2002–2003. The results of the integrated geophysical studies identified some areas with large thickness of subtrappean sediments and submitted in the form of a technical report (NGRI, 2004) to OIDB. Based on the results of this report, Directorate General of Hydrocarbons (DGH) would like to take up limited number of parametric drill holes in some of the identified areas. Reanalysis and modeling of the seismic data were undertaken along a few seismic lines identifying wide-angle reflection phases from top and bottom of the low velocity layer together with refraction data. 2-D travel-time inversion method of Zelt and Smith (1992) was used to derive the shallow velocity structure including the subtrappean sediment thickness and basement configuration along the profiles. The inversion method provides resolution and uncertainties of estimated model parameters along with the assurance that the data have been fitted according to a leastsquares norm and is capable of quickly analyzing the data while avoiding unnecessary structures (Zelt, 1999). The present paper deals with the refraction and wide-angle reflection data analysis
and processing results along the Jhagadia–Rajpipla (SW–NE) profile. The objectives of the present study are: (i) to determine the thickness of the sediments, Deccan Traps and subtrappean sediments; (ii) delineate the basement configuration providing with measures of resolution and uncertainty of the model parameters.
2. Geologic setting The study region is spread over 40,000 km2 area between the Narmada and Tapti rivers (Fig. 1) in western part of central India. The region is characterized by the Recent alluvium along the Tapti graben towards southeast, Archaean–Neoproterozoic granite gneiss and Paleo-Mesoproterozoic Aravalli/Delhi super group of rocks towards north, Meso-Neoproterozoic Vindhyan sediments towards east and the Cambay basin towards west. The Jhagadia– Rajpipla profile is located in the northwestern part of the Deccan syneclise and to the east of the Cambay basin. The Deccan volcanics represent an episode of volcanism marking the close of the Mesozoic era. The Deccan Traps were encountered at varying depths below the Tertiary sediments ranging in age from the Palaeocene to the Recent in the wells drilled in the Cambay basin (Roy, 1991; Tewari et al., 1995). Upper Jurassic to Middle Cretaceous Bagh and Lameta beds, representing the Mesozoic sediments underlying the Deccan Traps, is exposed to the east of the Cambay basin near Rajpipla (Fig. 1). The presence of sporadic outcrops of Jurassic and Cretaceous sediments on the margins of the Cambay basin lead to the possibility of a thicker marine sequence within the basin below the Deccan Traps. Deccan Traps also cover the whole of Narmada valley and adjoining areas with exposures of Mesozoic sediments towards north. The Deccan Traps show a marked thickening towards the west coast of India, suggesting that
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the center of eruption lies towards west (NGRI, 2004; Mahadevan, 1994). White and Mckenzie (1989) suggest that the major extrusion took place near the west coast of India around 65 Ma (Courtillot et al., 2000). Deccan Traps covered an area of 5,00,000 km2, excluding the portion down faulted into the Arabian Sea to the west. Mesozoic sediments belonging to the Bagh group are well developed in parts of the Narmada valley. These sediments are exposed mainly along the margins of the Deccan Traps, occupying a position between the Traps and the underlying Precambrian metamorphic rocks. The Mesozoic sediments were uplifted, faulted, intruded, and covered by the Deccan volcanic flows during the late Cretaceous.
3. Refraction experiment and the data analysis Seismic refraction data were acquired along the Jhagadia– Rajpipla profile from five different shot points using 120-channel Radio Frequency Telemetry (RFT) systems (Fig. 1). The data were recorded with 100 m receiver interval (10 Hz natural frequency) and 8–10 km shot interval on a spread length of 11.9 km using 2 ms sampling interval. The data from each shot point were recorded to a distance of 48 km (four spreads) to get an adequate coverage for basement configuration. The seismic recording system provided a wide dynamic range >110 db and a wide band of frequencies up to 125 Hz. In general, a shot hole drilled, to a depth of 20–25 m and loaded with 50 kg high energy explosive, was used as a seismic source. The charge size is increased in steps of 50 kg progressively as the shot-receiver distance increased. The large charges were accommodated in multiple holes, with either rectangular or triangular pattern of holes, with individual shot holes spaced at 10 m within the pattern. The data were recorded during the night to minimize cultural noise thereby increasing S/N ratio. The data were recorded simultaneously in the analog form on photographic paper and digital (SEG-D) form on magnetic tapes and converted to the SEG-Y format. Preprocessing includes trace editing, merging, frequency and velocity filtering using the ProMax seismic processing package on Sun work station. The seismic data are presented in the form of shot gathers (record sections) for various shot points to see the entire seismic data at a glance and identify the phases. Filtered record sections with a reduced travel-time are the best means of presenting the seismic refraction data. The seismic record sections plotted with a reduction velocity of 6 km s 1 are presented in Figs. 2a–2e. Amplitudes are trace normalized, which means that the maximum amplitude is the same for each trace. The records are band pass filtered in the frequency range of 10–24 Hz. In all the record sections (Figs. 2a–2e) Pi indicates the first arrival refraction and Pi the wide-angle reflection data. The overall signal-to-noise ratio of the first arrivals and reflection data as observed on record sections is good. The record sections for SPs 1, 2, 3, 4 and 5 are displayed in Figs. 2a–2e respectively. For the remainder of this paper, ‘‘offset” will refer to the distance between shot and receiver and ‘‘distance” will refer to a location along the profile relative to SP 1 at 0 km. Shot point number followed by SW or NE refer to data recorded to the southwest or northeast of the shot point, respectively. The record section of SP 1 shows only two refraction phases P1 and P2 (Fig. 2a) as the charge detonated was very low due to habitation. The record section of SP 2 (Fig. 2b) shows four refraction phases P1, P2, P3 and P5 as first arrival. From the apparent velocities of the phases (1.98–2.0 and 2.7–2.9 km s 1 respectively) and surface geology exposures, P1 and P2 represent phases from the layers of Recent and Quaternary sediments. The apparent velocities of P3 and P5 phases are 5.1 km s 1 and 6.15 km s 1. These orders of velocities are characteristic of Deccan Traps and granitic basement respectively in this region (Kaila et al., 1981). There is amplitude
decay at about 14 km offset from SP 2 NE (Fig. 2b) and also travel-time skip between the two refracted phases P3 and P5. The amplitude decay in the P3 refracted phase and the time skip in between the P3 and P5 phases indicate the presence of a low velocity zone (LVZ) beneath the Trap layer. This may be due to the fact that the relatively high frequency energy traveling in the relatively thin high velocity layer, overlying a low velocity zone, might have died out before the arrival of low-frequency energy from the basement. The low velocity zone overlying the basement might be responsible for the delay in the travel-time from the basement (Greenhalgh, 1977). The same phenomena of decay of refracted phase and travel-time skip are observed in SPs 3, 4 and 5 (Figs. 2c–2e). The energy of the first arrival refraction data from basement (P5) is relatively poor in most of the record sections probably due to loss of energy to the seismic wave while traveling in the Deccan Trap formation and LVZ. Different gains are used for different phases (low gain for near offset and high gain for far offset record sections) to minimize errors in data picking. Record sections of SP 2 NE, SP 3, SP 4 and SP 5 (Figs. 2b–2e) show wide-angle reflection phase designated as P4 immediately after the first arrival refracted phase P3. The reciprocal travel-times of this phase have been verified while identifying the phase. The prominent wideangle reflection phase immediately after the first arrival Trap refracted phase indicates existence of a large velocity contrast. The indication of a travel-time skip and amplitude decay in the refraction data together with the presence of a prominent wide-angle reflection phase due to large velocity contrast strengthens the assumption of a low velocity zone under the Trap cover. This prominent reflection phase has been identified as a reflection from the interface between the Trap and the LVZ. Similarly there is another reflection phase designated as P5 and identified in SP 4 and SP 5 based on signal strength as there is no reciprocal coverage. This has been interpreted as wide-angle reflection phase from the bottom of the LVZ. It is observed that in the record section of SP 2 (Fig. 2) the refraction phase P3 from the high velocity Deccan Trap has been recorded at 5 km offset from the shot point 2 NE, while the refraction phase P2 recorded up to 10 km towards SW do not show the high velocity refraction phase P3. This indicates that the high velocity Deccan Trap layer must have gone down in SW direction. It is also seen that the offset distance of the refraction phase from basement (P5) (Figs. 2b–2e) decreases progressively towards northeast indicating that the basement depth is shallow towards northeast and deepens towards southwest. The travel-time skip between P3 and P5 phases in the record section of SP 5 (Fig. 2e) is at less than 5 km offset and the magnitude of the skip is small. This indicates that the basement is at very shallow depth near SP 5 and the layers above basement are thin. Again there is a travel-time skip in the basement refracted phase (P5) at about 25 km offset in the record section of SP 5 (Fig. 2e) indicating deepening of the basement towards SW. Similarly Amplitude decay in the Trap refracted phase (P3) and very large travel-time skip between the refracted phases P3 and P4 in the record sections of SP 3 SW and SP 4 SW might be due to the combined result of LVZ beneath the Deccan Traps and deepening of the basement towards southwest. The variation in the magnitude of the skip between the refracted phases P3 and P5 indicates the thickness variation in the LVZ (higher the magnitude larger the thickness) as can seen in the record sections of SP 3 SW and SP 3 NE; SP 4 SW and SP 4 NE. Based on the above observations and also the exposures of Mesozoic sediments near Rajpipla towards the NE of the profile, a low velocity layer correspond to Mesozoic sediments was inferred to lie above the basement and below the Deccan Trap along the profile. As seen from the record sections, the first arrival refraction data (P3) representing the Deccan Trap terminates and the first arrivals at greater offsets are delayed. The termination of diving wave from Deccan Trap occurs when the wave feels the base of
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Fig. 2a. The record section of SP 1 of the profile shows the P1 and P2 refraction phases.
the Trap, consistent with Snell’s law, is refracted into the lowvelocity subtrappean sedimentary section. The offset at which
the direct-wave termination occurs is a function of the velocity structure and thickness of the Trap layer. So the Deccan Trap diving
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Fig. 2b. The record section of SP 2 of the profile shows the P1 and P2 refraction phases from Recent and Quaternary and P3 and P5 refraction phases from Deccan Trap and basement respectively. Amplitude decay for the P3 phase is observed at 14 km distance. The travel-time ‘skip’ between P3 and P5 indicates the presence of a low velocity zone (LVZ) beneath the Trap layer. The absence of P3 towards SW and its presence at 5 km offset towards NE indicates deepening of the Trap rock towards southwest. Wideangle reflection phase P4 from top of the LVZ is shown.
Fig. 2c. Same as Fig. 2b for SP 3 showing refraction phases P1, P3 and P5 with travel-time skip between P3 and P5 phases. Wide-angle reflection phase P4 from top of the LVZ is shown.
wave can be used to image the Trap velocity structure. Trap thickness will be obtained from the maximum depth penetration of the diving wave that fits the termination point of the Trap diving wave (Tewari et al., 1995). Again the wide-angle reflection (P4) further constrains the Trap layer thickness and top of the LVZ, thus eliminating the ambiguity involved in identification of the Trap termination point in the refraction data. Similarly the sedimentbasement interface will be constrained using the correct arrival
time evident at large shot-receiver offset and the velocity of the LVZ (Jarchow et al., 1994; Tewari et al., 1995) together with the wide-angle reflection data (P5) from the bottom of the LVZ. 4. Methodology In the present study, first arrival P-wave refraction and wide-angle reflection data are modeled for shallow crustal velocity
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Fig. 2d. Same as Fig. 2b for SP 4 showing refraction phases P1, P3 and P5 with travel-time skip between P3 and P5 phases. Wide-angle reflection phases P4, P5 from top and bottom of the LVZ are shown.
Fig. 2e. The record section for SP 5 showing refraction phases P2, P3 and P5 with travel-time skip between P3 and P5 at 5 km offset indicates presence of LVZ. The skip in the travel-time of basement refraction P5 at about 25 km offset indicates deepening of the basement towards southwest. Wide-angle reflection phases P4, P5 from top and bottom of the LVZ are shown.
model using the travel-time inversion (Zelt and Smith, 1992) method. The inversion scheme is based on model parameterization and a method of ray tracing suited to the forward step of the inverse approach. The model was parameterized by linear interpolation between an irregular grid of depth nodes and upper and lower layer velocity nodes. A smooth layer boundary simulation is used to avoid scattering and focusing of ray paths and to stabilize the inversion. The travel-times and their partial derivatives with respect to velocity and depth nodes are calculated during the ray tracing that uses an efficient numerical solution (Zelt and Ellis,
1988) of the ray tracing equations (Cerveny et al., 1977). The calculated response of the model is compared with the observed data, and the model parameters are updated using the correction vector obtained from the damped least-squares inversion (Zelt and Smith, 1992; Zelt, 1999). The process is repeated until we achieve a satisfactory fit corresponding to a normalized v2 value of nearly 1. However, it is not always possible to obtain v2 = 1 while maintaining acceptable resolution values of model parameters because of data sampling at small-scale heterogeneities that cannot be resolved by modeling.
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5. Modeling procedure We have analyzed the first arrival refraction and wide-angle reflection data to derive the basement configuration and overlying Deccan Trap and sedimentary formations along the profile. Based on the geological information and considerable change in slopes of the refraction phases, first arrival refraction and wide-angle reflection data are picked from record sections of various shot points using the SeisWide software package. The dominant period of the data is around 50 ms, which has been used as the picking uncertainties to the data. This uncertainty assignment is subjective and is based on one period of dominant cycle of the data, as has been used by Sain et al. (2000). The uncertainty of picking seismic data also depends on the signal-to-noise ratio of each phase. However, utmost care has been taken while picking both refraction and reflection data using varied gains for various phases to avoid ambiguous picks. The first arrival refraction data has been plotted on the record sections (Fig. 2) by dots (). There is clear indication on the shot gathers of shot points 2–5 (Figs. 2b–2e), sudden terminations in the Trap diving wave (P3). The offset at which this termination occurs is approximately 14 km in SP 2 (Fig. 2b). This termination point appears to vary substantially from one record section to the other. While modeling the refraction data, we have considered Pwave velocity of 3.5 km s 1 to the low velocity zone (Mesozoic sediments) present beneath the Trap layer. This velocity was obtained from the available well data and from the micro spreads recorded on the Mesozoic exposures in Cambay, Saurashtra and Kutch basins for velocity determination. A uniform velocity gradient of 0.1 km s 1 km 1 for high velocity Trap layer was taken on the basis of the earlier studies in Saurashtra and Kutch basins, since both the gradient and thickness of the layer play an important role in causing the travel-time shift (Tewari et al., 1995). The first arrival refraction data from the sediments, Deccan Trap, basement layer and wideangle reflection from top and bottom of the LVZ are input to the travel-time inversion program RAYINVR (Zelt and Smith, 1992). The modeling approach is one of ‘layer stripping’, whereby successively deeper layers are determined by keeping fixed the velocity and structural information of overlying layers. Initially, the 1-D velocity-depth functions were derived separately for each shot point using the method of Sain and Kaila (1996). An initial 2-D velocity structure was then prepared along the profile by smoothly joining the individual shot point results for further refinement. The travel-times of the first segment (P1) of each shot point at profile distances of 0.8, 9.78, 16.87, 26.28, and 37.00 km along the profile are inverted simultaneously using the 2-D inversion scheme of Zelt and Smith (1992). This determined the velocity variation of the first layer (1.95–2.3 km s 1). The data are fitted with an rms travel-time residual of 56 ms corresponding to a normalized v2 value of 1.260. The interface structure of the first layer (0.30–0.70 km) and velocity variation of the second layer (2.70– 3.05 km s 1) are then calculated by inverting the travel-time data of the second segments of all shots by holding fixed the velocity variation of the first layer already derived. The data are fitted with an rms travel-time residual of 52 ms corresponding to a normalized v2 value of 1.207. The first two layers may correspond to the Recent and Quaternary sediments. To find out the thickness of the third layer (Trap thickness) it is important to pick precisely the diving wave (P3) termination point. After picking the termination points for all the shot points, rays were traced from individual shot points to their respective termination points and the base of the trap layer was determined from the maximum depth penetrated by these rays with a P-wave velocity of 4.8–5.1 km s 1. Then wide-angle reflection data (P4) were used to generate the computed travel-time from the bottom of the Trap layer. Thus the base of the trap layer would be the top of the low velocity zone (3.5 km s 1), has been obtained (0.9–3.9 km) by modeling both
the refraction (P3) and reflection (P4) data. The data are fitted with an rms travel-time residual of 38 ms and normalized v2 value of 0.588 for refraction and rms travel-time residual of 47 ms and normalized v2 value of 0.877 for reflection data. After the Trap velocity and thickness were determined, a simultaneous inversion for basement velocity and LVZ-basement interface position (bottom of the LVZ) was carried out using the refraction data (P5) at large offset and also wide-angle reflection (P5) data. The uncertainty of the basement depth derived from the refraction data due to ambiguity in picking at few offsets in some shot points along the profile has been reduced to a great extent with the reflection data modeling. The basement velocity (5.90–6.15 km s 1) and depth (1.2–4.9 km) variation were obtained with an rms travel-time residual of 48 ms and normalized v2 value of 0.916 and 0.947 for refraction and wide-angle reflection data respectively. To see the variation of the basement depth with velocity of the LVZ, we have modeled the data set assuming a velocity of 4.3 km s 1 for the low velocity sediment layer. It is observed that the depth to the basement is increased by about 500 m with increase in the velocity of LVZ. For the inversion we use the overall damping factor of 1.0, with apriori velocity uncertainty of 0.1 km s 1 and depth uncertainty of 0.1 km. The rays traced through the final velocity model and their travel-time fit are shown in Figs. 3a and 3b. The final model along the profile is displayed in Fig. 4. The model along the profile corresponds to an overall rms travel-time residual of 47 ms and normalized v2 of 0.899. The number of rays traced through the final model, rms travel-time residual and normalized v2 value for various phases corresponding to all shots is shown as modeling results in Table 1. The numerical values of velocities of various layers (Fig. 4) show the lateral variation in velocity. The velocity model is derived based on the ability to trace rays through the final model to almost all observation points and a trade-off between achieving a sufficiently small travel-time residual of the order of data uncertainties and an adequately high parameter resolution. Adding more nodes will typically reduce the travel-time residual, but decrease the parameter resolution. Therefore, a velocity model with few parameters defining a minimum structure model is generally sought. This represents the true structure (O’Leary et al., 1995) and results are free from modeling artifacts. We decided the number of nodes on the basis of the observed and computed traveltime fit within the data uncertainties and avoiding unnecessary structures to the boundary interfaces. The model parameters used in the inversion are shown in Fig. 5.
6. Resolution and uncertainty The most important aspect of model assessment is to provide a measure of resolution and uncertainties of estimated model parameters from the diagonal elements of the resolution and covariance matrices respectively. Generally, resolution values range from 0 to 1 and depend on the relative number of rays sampling each model parameter. The desired fit and resolution are attained within 3–4 iterations with most of the velocity and depth nodes attain resolution greater than 0.8 along the profile (Table 2) indicating that the model is well resolved. To account for the nonlinearity of travel-time inversion and to provide significant insight into the model constraint, a uni-parameter uncertainty test (Zelt and Smith, 1992; Zelt, 1999) is carried out at few selected nodes to provide the upper bounds. For obtaining absolute uncertainty of a model parameter, we perturb its value from that in the final model and hold it fixed while inverting the observed data involving all other model parameters that were determined along with the perturbed parameter in the final model. Increase or decrease in perturbation is continued until the final model fits the observed data. The maximum perturbation of the parameter that allows a
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Fig. 3a. Refraction and reflection rays (top) traced through the final velocity model for shot points 1, 2 and 3 along the profile. Comparison of observed (vertical bars) and theoretical travel-times (line) for various refracted and reflected phases (bottom). The data are plotted in reduced time using a reduction velocity 6.0 km s 1.
comparable fit to the observed data gives an estimate of its absolute uncertainty. Fig. 6 displays the absolute uncertainty of the
velocity node of the basement layer at 20 km and boundary depth of the basement interface at 10 km profile distance along the
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Fig. 3b. Same as Fig. 3a for shot points 4 and 5 along the profile.
profile. This shows the absolute velocity uncertainty lying between 5.75 and 6.0 km s 1 ( 0.15 km s 1 to +0.10 km s 1) and depth uncertainty between 4.68 km and 4.98 km ( 0.18 km to + 0.12 km) corresponding to 50 ms travel-time residual. We have carried out the uncertainty test at few selected nodes to save the modeling time and observed that the overall velocity and depth uncertainties are of the order of ±0.12 km s 1 and ±0.15 km respectively.
7. Results and discussion The basement configuration along the seismic profile has been derived to a depth of about 5.0 km by travel-time inversion of first arrival refraction and wide-angle reflection data (Fig. 4). Alluvium (Recent) outcrops the entire length of the profile and has been recorded as direct wave in the record sections of SPs 1, 2, 3 and 4. There are four layers above the basement. All the layers including the basement show maximum depth below the shot point 1 near Jhagadia in the southwest and shallowing to minimum depth towards northeast near Rajpipla. The first layer P-wave velocity is
ranging between 1.95 and 2.3 km s 1 represents Recent (alluvium) as the formation is exposed along the profile. The thickness of this layer is 300 m near Jhagadia attains maximum thickness of 700 m near SP 2 and gradually thins out towards northeast and disappears near shot point 5. Second layer with a velocity of 2.7–3.05 km s 1 corresponds to the Quaternary sediments, which exhibits maximum thickness of about 2900 m below shot point 1 near Jhagadia. The thickness of this layer rapidly decreases between SP 2 and SP 3 reaching to a value of about 150 m near Rajpipla to the northeast. The third layer with a velocity of 4.8–5.1 km s 1 represents the Deccan Trap formation. The maximum thickness of the Trap layer is 1000 m near SP 3 thinning to about 500 m on either side. Top of the Trap layer is steeply deepens between shot points 2 and 3 because of which, there is huge thickness of Quaternary sediment deposition over the Trap layer southwest of SP 2 near Jhagadia. The steep deepening of the Trap layer between shot points 2 and 3 can be inferred as a fault (Fig. 4). The amplitude decay and travel-time skip in the first arrival refraction data seen in the shot points 2, 3, 4 and 5 (Figs. 2b–2e) indicate the presence of a low velocity zone (LVZ) beneath the Traps. The
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Fig. 4. The P-wave velocity model along the profile. The numbers within the model represent localized velocities (km s 1) of various layers indicating the lateral variation of the velocities. Velocities of the top two layers are 1.95–2.30 and 2.70–3.05 km/s respectively. Inverted triangles at the top show the location of shot points.
Table 1 Modeling results along the profile. Phase
No. of observations picked
Data uncertainty (ms)
RMS residual (s)
Normalized v2 value
No. of rays traced through model
Refra-P1 Refra-P2 Refra-P3 Refra-P5 Reflec-P4 Reflec-P5
104 82 197 239 169 101
50 50 50 50 50 50
0.056 0.052 0.038 0.048 0.047 0.048
1.260 1.207 0.588 0.916 0.877 0.947
98 82 195 239 167 96
Number of data points used: 877. RMS travel-time residual: 0.047. Normalized chi-squared: 0.899.
Fig. 5. Velocity (dots) and depth (squares) nodes representing the model parameterization along the profile.
thickness of low velocity Mesozoic sediments is about 1000 m near Jhadadia at SP 1 and increases to maximum thickness of 1700 m near SP 2. The low velocity sediment thickness at 25 km distance is about 1100 m and thereon gradually shallows to 300 m towards northeast near Rajpipla. The basement with a velocity ranging between 5.9 and 6.15 km s 1 is at a depth of 4.9 km below SP 1 near Jhagadia. The basement depth decreases from 4.6 km near shot point 2 to 2.6 km near shot point 3 and further shows an upward trend, reaching to a value of 1.2 km near Rajpipla towards northeast. The presence of thick low velocity sediments and steep deepening of the basement interface between shot points 2 and 3 (2 km throw) is reflected on record sections of SP 4 SW in the
form of amplitude decay and time skip between P3 and P5 phases. Again record section of SP 5 SW indicates time skip at 25 km offset in the basement refracted phase P5. Based on the refraction data of SP 4 SW and SP 5 a fault can be inferred in the basement between SP 2 and SP 3. The velocity model derived in the present study delineates the Recent, Quaternary, Trap, low velocity Mesozoic sediment thickness and depth to the basement. It also provides resolution and uncertainty values to the model parameters. The profile is crossing Sinor-Valod (N-S) profile near SP 3 and Mavli-Gulkumar (N-S) profile near SP 5. The results of these profiles (NGRI, 2004) nearly correlates the results obtained in the present profile at the cross points. The inferred fault between SP 2 and 3 in the Deccan Trap layer might have played significant role in deposition of the sediments over it. Sastry et al. (1964) opine that the Cambay basin appears to have been formed due to faulting of the Deccan Trap which incidentally also controlled the nature of the structure in the overlying sediments. The Broach–Ankleswar graben, derived from the deep seismic sounding studies along the Mehmadabad– Billimora profile (Kaila et al., 1981) in the Cambay basin, is located to the west of the present profile. The DSS results of the Broach– Ankleswar graben indicate thick Tertiary sediments deposited over an irregular surface of the Deccan Trap. The granitic basement is at a depth of 5.2 km and there is a possible occurrence of low velocity Mesozoic sediments of 1.2 km thick beneath the Deccan Trap layer (Kaila et al., 1981). In the present analysis the basement depth of 4.9 km inferred in the southwest of Jhagadia nearly coincides with
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Table 2 Resolution estimates along the profile. Distance (km) Velocity L1 0.0 L1 10.0 L1 20.0 L2 5.0 L2 10.0 L2 40.0 L3 16.0 L3 26.0 L5 20.0 L5 27.0 Depth L2 2.0 L2 5.0 L2 12.0 L2 16.0 L2 18.0 L2 20.0 L3 12.0 L3 14.0 L3 16.0 L3 25.0 L3 30.0 L3 40.0 L4 12.0 L4 20.0 L4 25.0 L4 30.0 L4 40.0 L5 18.0 L5 20.0 L5 25.0 L5 30.0 L5 35.0
Velocity nodes (km/s)
Velocity resolution
Standard deviation
2.00 1.98 1.96 3.05 2.90 2.90 4.81 4.87 5.90 6.00 Depth nodes (km)
0.81 0.96 0.87 0.82 0.51 0.88 0.55 0.59 0.86 0.89
0.044 0.021 0.036 0.042 0.070 0.035 0.067 0.064 0.038 0.033
0.50 0.68 0.70 0.50 0.40 0.35 1.90 1.50 1.00 0.60 0.50 0.15 2.94 1.77 1.43 1.36 0.88 2.66 2.55 2.55 2.16 1.72
0.82 0.86 0.83 0.61 0.61 0.74 0.79 0.73 0.81 0.91 0.91 0.84 0.75 0.75 0.76 0.64 0.60 0.70 0.80 0.84 0.82 0.87
0.042 0.037 0.042 0.062 0.062 0.052 0.046 0.052 0.044 0.030 0.030 0.040 0.050 0.050 0.049 0.060 0.063 0.055 0.045 0.040 0.042 0.036
(L = layer)
the basement depth estimates in the earlier DSS profile. The low velocity Mesozoic sediments in the present profile are about 1.7 km thick near the shot point 2. The Mesozoic sediments in western India occupy a large portion of Rajasthan, Gujarat and Madhya Pradesh and are mostly marine, thus attracting the attention of the oil industry. On the southeastern margin of the Cambay basin and western part of the Deccan syneclise near Rajpipal, a 600 m thick section of the Cretaceous Bagh beds (Mesozoic) is exposed (Fig. 1). Poddar (1964) had earlier suggested that there is likelihood for the presence of suitable facies and of favorable prospects of hydrocarbon-bearing Mesozoic sediments below the Deccan Traps towards the Ankleswar oil field. From these considerations and the fact that the existence of a relatively thin Trap and about 1.7 km thick Mesozoic sediments near SP 2 in the present profile is significant. Deep seismic sounding (DSS) studies across Narmada–Son lineament mapped Deccan Trap thickness and also inferred subtrappean sediments in the Narmada–Tapti region (Kaila, 1988; Sridhar and Tewari, 2001). The present study area lies in between two DSS profiles along Mehmadabad–Billimora in the west and Thuadara–Sindad towards the east. Along the Thuadara–Sindad profile the thickness of the Deccan Trap is found to be around 900 m and south of Sendhwa up to Tapti river the thickness of Mesozoic sediments is around 1.9 km (Kaila, 1988). Sridhar and Tewari (2001) in their reinterpreted seismic results along Thuadara–Sindad profile delineated a graben between the rivers Narmada and Tapti. Within the graben, depth to the basement is 5000–5500 m between Sendhwa and Tapti and contains 1000–2800 m thick Mesozoic sediments under a thick (2000–2700 m) cover of the Deccan Trap. From the
Fig. 6. (a) Root-Mean-Square (rms) travel-time residual as a function of velocity perturbation for the velocity node of the basement layer at 20 km profile distance. The absolute velocity uncertainty lies between 5.75 and 6.0 km s 1 ( 0.15 to +0.1 km/s) corresponding to 50 ms travel-time residual. (b) Root-Mean-Square travel-time residual as a function of depth perturbation for the depth node of the basement interface at 10 km profile distance. The absolute depth uncertainty lies between 4.68 and 4.98 km ( 0.18 to +0.12 km) corresponding to 50 ms travel-time residual.
Isopach contour maps of the Mesozoic sediments in the Narmada– Tapti region, Kaila (1988) hypothesis a Mesozoic basin in the Narmada–Tapti area formed a part of larger Mesozoic sea that extended in the shape of an arc from Sanawad-Mahan through Barwani-Sindad, Ankaleswar-Surat, Navibandar-Amreli, Kutch, Rajasthan, Sind and up to Salt Range. The Trap and subtrappean sediment thickness derived in the present study along the profile in the Narmada–Tapti region of the western Deccan syneclise nearly correlates with the Trap and Mesozoic sediment thickness derived in the Narmada–Tapti region by Kaila et al. (1981) and Kaila (1988) from the DSS data and confirm the existence of Mesozoic basin in the Deccan syneclise region.
8. Conclusions The present study images the hidden low velocity Mesozoic sediments lying below the high velocity Deccan Trap volcanic rocks in the Narmada–Tapti region of the western part of Deccan syneclise. The velocity model derived along the profile is well constrained providing model parameter resolution and uncertainties. The first two layers with P-wave velocities of 1.9–2.3 km s 1 and 2.7–3.05 km s 1 are Recent and Quaternary sediments with a thickness variation of 300–700 m and 150–2900 m respectively. The Trap layer (velocity 4.8–5.1 km s 1) thickness varies from 500 to 1000 m and the low velocity Mesozoic sediment thickness ranges from about 300 m to 1700 m along profile. Travel-time skip and amplitude decay of the first arrival refraction together with the wide-angle reflection data from top and bottom of LVZ have been used to derive the thickness of the low velocity zone. The basement (5.9–6.15 km s 1 velocity) lies at a depth of 4.9 km approximately near Jhagadia and shallows to about 1.2 km near Rajpipla. The thick low velocity Mesozoic sediments present below the relatively thin Deccan Traps near SP 2 along the profile is an important zone
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